1. Field of the Invention
This invention relates generally to impedance matching and impedance transformation, and more particularly to differential impedance matching and transformation.
2. Background Art
Impedance matching circuits generally are utilized to efficiently transfer energy at a junction point where electronic circuits having different characteristic impedances are connected to each other. This is accomplished by rendering the impedances seen on either side of the junction point identical, that is, to match line impedances and load impedances of the circuits.
Such line impedance matching is necessary not only for a wire terminal but also for a wireless terminal, wherein the impedances are matched at 50, 75 and 100 Ohms according to convention and the characteristics of the antenna and transmission lines. For example, radio frequency (RF) circuits often utilize a low noise amplifier (LNA) to amplify a received signal without adding significant noise. The performance of the LNA depends on the impedance of the circuit coupled to the LNA input. Generally, an LNA is designed to perform optimally while also providing a good impedance match. However, when the impedance is not matched, the performance, such as output power, efficiency, linearity, etc., of the LNA is degraded, as illustrated in FIG. 1.
FIG. 1 shows a prior art RF receiver chain 100 wherein signal reduction is experienced through mismatched impedances. The prior art RF receiver chain 100 includes an antenna 102, which provides a signal to an LNA via an LNA interface 106. The LNA is the first component in the receiver chain 100 to process incoming signals, after the antenna 102 and an RF filter. In order to keep the system sensitivity high, the LNA should receive as much of the signal as possible, which requires the LNA impedance to be matched to the antenna 102 impedance.
For example, in FIG. 1, the antenna 102 impedance is 50 Ohms. If the LNA impedance does not match the antenna 102 impedance of 50 Ohms, part of the signal 110 will “bounce” off and radiate back out of the antenna 102. As a result, the signal transfer 108 will be reduced and emissions problems may occur if the reflection is too large.
Hence, components are often added between the LNA and the antenna 102 to ensure that the impedances of the LNA and the antenna 102 match, as illustrated in FIG. 2. FIG. 2 is a block diagram showing an RF receiver circuit 200 utilizing discrete components. In particular, the RF receiver circuit 200 includes an antenna 102 coupled to an RF filter circuit 202. The RF filter circuit 202 includes a balun to provide a differential input to a discrete match circuit 204, which provides a differential input to the LNA 206 residing on a chip 208.
The discrete match circuit 204 utilizes discrete components, such as inductors, to match the impedance of the antenna 102 to the LNA 206. Unfortunately, discrete match components add extra cost and occupy valuable board space. For a differential LNA as illustrated in FIG. 2, two inductors are needed, which further increases the costs and the space required for the inductors.
In an attempt to reduce costs and save valuable board space, match inductors have been placed on the chip 208, as shown in FIG. 3. FIG. 3 is a block diagram showing an RF receiver circuit 300 utilizing in-silicon matching via inductors. Similar to FIG. 2, the RF receiver circuit 300 includes an antenna 102 connected to an RF filter circuit 202. However, the RF circuit 300 replaces the discrete match circuit of FIG. 2 with in-silicon inductors 302, which are incorporated on the chip 208 at the input of the LNA 206.
Although the two in-silicon inductors 302 do not occupy board space, unfortunately, the two in-silicon inductors 302 occupy a significant amount of silicon to achieve the required inductance. Also, a loss is associated with each in-silicon inductor 302, which is proportional to the metal length.
In view of the forgoing, there is a need for techniques for improved impedance matching and transformation. The impedance matching and transformation techniques should require less area and result in lower loss, thereby improving the noise figure.